Feasibility of Hydrogen Production from Cellulose and Prediction of the Product Distribution: Thermodynamics Analysis

High carbon emissions, depleting fossil energy reserves have become a global problem. It is necessary to develop renewable energy sources that are environmentally friendly. Hydrogen (H2) is one of the energy sources and carriers that can be developed. This gas can be produced from renewable, sustainable, and economical resource such as biomass that contains cellulose as the main ingredient. This thermodynamic analysis of H2 production from cellulose is necessary as a theoretical study to determine the feasibility of the reaction. The computational thermodynamic was analyzed using Microsoft Excel 2019 and Matlab Program R2013a. Prediction of the equilibrium composition of the substances involved in the reaction was attempted by minimization Gibbs free energy change with Lagrange undetermined multipliers methods. As a result, the value of ΔHr 0; ΔSr 0 and ΔGr 0 are +624,7500 kJ/mol; +2,1491 kJ/mol.K and; -26,1540 kJ/mol, respectively. Analysis of equilibrium constant of this conversion has a large ln K value (> 1). A negative ΔGr 0 value and large ln K indicates that the formation of H2 from (C6H10O5)n is plausible and feasible and reaction product formation is strongly favored at equilibrium. The composition of the substances involved at 298 K from the largest to the smallest is CH4 (4.5 mol), H2O (3 mol), CO2 (1.5 mol), H2 (1.28×10 -5 mol), HCOOH (5.85×10-10 mol), C6H12O6 (3.72×10-10 mol) and C6H12O5 (1.35×10 -10 mol). Interestingly, H2 yield will rise significantly with the increase of reaction temperature. This preliminary study provides an overview of reaction conditions so that H2 production from biomass can be produced maximally.

; ΔS r 0 and ΔG r 0 are +624,7500 kJ/mol; +2,1491 kJ/mol.K and; -26,1540 kJ/mol, respectively. Analysis of equilibrium constant of this conversion has a large ln K value (> 1). A negative ΔG r 0 value and large ln K indicates that the formation of H 2 from (C 6 H 10 O 5 )n is plausible and feasible and reaction product formation is strongly favored at equilibrium. The composition of the substances involved at 298 K from the largest to the smallest is CH 4 (4.5 mol), H 2 O (3 mol), CO 2 (1.5 mol), H 2 (1.28×10 -5 mol), HCOOH (5.85×10 -10 mol), C 6 H 12 O 6 (3.72×10 -10 mol) and C 6 H 12 O 5 (1.35×10 -10 mol). Interestingly, H 2 yield will rise significantly with the increase of reaction temperature. This preliminary study provides an overview of reaction conditions so that H 2 production from biomass can be produced maximally.

INTRODUCTION
The decline in environmental quality due to carbon emission has become a global problem. Based on the Statistical Review of Energy released by BP in 2020, the trend of carbon emission is showed where the average annual growth in carbon emissions during 2018 and 2019 is greater than the 10-year average. The industrial and transportation sectors are the largest energy consumers and contribute to the increase in greenhouse gases. This is because the primary energy source currently used is fossil energy (petroleum, natural gas, and coal) (BP 2020). The use of renewable energy sources that are environmentally friendly is absolutely necessary not only because of high carbon emissions but also depleting fossil energy reserves. Hydrogen (H 2 ) is one of the energy sources and carriers that can be developed. H 2 has high chemical energy per mass (122 kJ/g) (Dincer & Zamfirescu 2016), superior to that of many fossil fuels, and its combustion only results in water and zero carbon (Singh & Dutta 2018). This diatomic gas can be produced from renewable, sustainable, and economical resources such as plant crops, agricultural residues, and woody biomass (Lan et al. 2020). Biomass contains lignocellulose that mainly consists of three polymers which are cellulose (>40%), hemicellulose (20%-40%), and lignin (5%) (Kuehnel & Reisner 2018;Megashah et al. 2018).
Hydrogen can be produced from cellulose (C 6 H 10 O 5 )n in several thermochemical processes such as steam gasification (>1000K), fast pyrolysis (450-650K), thermo-catalytic reforming which is associated with the problems of catalyst deactivation and by-product (e.g. CO) production and hydrolysis by acids or enzymes (Nikolaidis & Poullikkas 2017;Takise & Sekine 2019). Due to high energy intensity and difficulties in H 2 separation and purification from mixture yields, nowadays, preferred H 2 production is from photocatalytic reforming process (photoreforming) (Hao et al. 2018;Lan et al. 2020). This process offers more sustainable and environmentally friendly technology since it employs solar as energy source and photocatalyst to accelerate the reaction. Cellulose can be convert to H 2 according to the following reactions (Hao et al. 2018): (C 6 H 10 O 5 ) n (s) + H 2 O (l) nC 6 H 12 O 6 (l) (1) Therefore, the net reaction will be Some researchers doubt this process can be used for effective H 2 production. This is maybe because of the rigid intra and inter-molecular hydrogen bonds in cellulose, it is difficult to hydrolyze cellulose to platform organic molecules and then converting it to H 2 . Hence, thermodynamic analysis of H 2 production from cellulose is necessary to determine the feasibility of the reaction. Some of the existing thermodynamic studies generally use platform organic compounds (Huang et al. 2020) such as methanol, ethanol, glycerol, and monosaccharides such as glucose. Thermodynamic studies of polysaccharides have not been carried out and existing studies use the glucose molecule as a model because it is a major derivative of cellulose.
This paper discusses the thermodynamic analysis of cellulose reforming to H 2 in the absence of a photocatalyst. This analysis will be a preliminary study to assess the feasibility of this reaction. Using the computational method, this theoretical study can be used to predict the conditions of the cellulose reforming reaction and the distribution of the products obtained.

COMPUTATIONAL THERMODYNAMIC ANALYSIS
This thermodynamic study was attempted by means of a computational procedure using Microsoft Excel 2019 and Matlab Program R2013a. The analysis consists of several steps, namely: (1) calculating changes in reaction for both standard enthalpy (ΔH o r ) and entropy (ΔS o r ), and then the change in free energy of reaction Gibbs (ΔG o r ); (2) determine the reaction equilibrium constant (K) at various temperatures (3) determine the change in total Gibbs free energy (ΔG t ); (4) minimization of Gibbs energy and prediction of the equilibrium composition of the substances involved in the reaction at the specified temperature and pressure. This minimization introduces Lagrange undetermined multipliers methods and computed using MatLab R2013a program.
For the first step, we calculate ΔH o r ; ΔS o r and ΔG o r according to (5), (6) and (7) (5) The equilibrium constant (K) determined based on (8): Or by Van't Hoff equation: that can be rewrite as (10) with ΔH r 0 is standard enthalpy; T is reference temperature of reaction (298 K); and T' is determined temperature of the reaction (This thermodynamic analysis take place at T=303-353 K). K' is reference of equilibrium constant. The value of ln K can be plotted to T.
Further thermodynamic analysis step aims to calculate the equilibrium composition by minimizing the free energy of Gibbs. The Gibbs energy minimization is limited or constrained by the number of moles of the substance (atoms) involved in the reaction (atomic balance constraints) (Khonde et al. 2021;Tsanas et al. 2017).
The total Gibbs free energy change can be calculated by (11). (11) where (12) and for gases compounds (13) Hence, the total Gibbs free energy change from the mixture can be written as (14) Subsequently, (14) is the objective function that will be minimized. In a closed system, the number of moles of each atom is fixed during a chemical reaction. The total number of moles of a particular atom present in all chemical species in each chemical reaction must equal the initial mole number of that atom.
which k = 1,2,3,… Or which k = 1,2,3,… Minimization was done by substituting boundary conditions in the objective function. The boundary condition is the number of atoms involved in the reaction based on the proposed mechanism, namely three atoms including C, H, and O. The number of moles of the three atoms is entered in the objective function using the Lagrange Multiplier: λ 1 , λ 2 and λ 3 . Therefore (14) will be: (17) where G t is the total Gibbs free energy; G i 0 is standard Gibbs free energy of species (i); R is the molar gas constant; T is the temperature of the system; f i is fugacity of species (i) in the system, f i 0 is standard state fugacity of species (i) and n i is the number of moles of species (i).
where = 0 + and for gases compounds formation, destruction and rearrangement of chemical bonds of compound involve in the reaction, so there will be a change in energy of the system. The change of energy can be observed from ΔH o r ; ΔS o r ; and ΔG o r . Standard enthalpy change can be useful for determining the temperature effect on the equilibrium constant of H 2 production and thus on H 2 yield. In addition, Gibbs free energy changes can be useful for determining the existence of chemical equilibrium in the system and the influence of process variables on the H 2 yield (Hill & Root 2014). One of the important things in thermodynamics analysis is the availability of thermodynamics data sources. In this analysis, the data is tabulated in Table 1   From (5), (6) and (7), the value of ΔH r 0 ; ΔS r 0 and ΔG r 0 are +624,7500 kJ/mol; +2,1491 kJ/mol.K and -26, 1540 kJ/mol, respectively. Even though the value of ΔH r 0 is positive, the value is too small compared to the positive value of ΔS r 0+ . This resulted in the negative value of ΔG r 0 . With a negative ΔG r 0 value, the reaction to form H 2 from (C 6 H 10 O 5 )n can take place spontaneously under standard conditions (298 K, 1 atm). The reaction takes place exothermically (Hua et al. 2020;Smith et al. 2018). This negative value of ΔG r 0 also indicate that the conversion of cellulose can be accelerated using a catalyst.  A large ln K value (> 1) indicates that the reaction, the formation of H 2 from (C 6 H 10 O 5 ) n can be plausible and feasible. With a negative ΔG r 0 value and large ln K, the product formation at equilibrium is strongly favored at equilibrium (Cheng et al. 2019;Muhammad et al. 2017). The higher the temperature, the greater the value of ln K, which indicates an infinite equilibrium.

PRODUCT DISTRIBUTION ANALYSIS
Based on the possible reaction mechanisms as stated by Hao et al. (2018) and Zou et al. (2018), the substances involved in the reaction is served in Table 3 that consists of cellulose ((C 6 H 10 O 5 ) n ), water (H 2 O), glucose (C 6 H 12 O 6 ), formic acid (HCOOH), carbon dioxide (CO 2 ), hydrogen (H 2 ) and methane (CH 4 ).  The value of number of atomic mass k in the system (A k ) that is required can be determined from the initial mole numbers, and the number of atom k per molecule i(a ik )values that comes directly from the chemical formulas of each species in the reaction (as shown in Table 4). Based on computational analysis, the composition of the substances involved in (reaction temperature 298 K from the largest to the smallest are CH 4 (4.5 mol), H 2 O (3 mol), CO 2 (1.5 mol), H 2 (1.28×10 -5 mol), HCOOH (5.85×10 -10 mol), C 6 H 12 O 6 (3.72×10 -10 mol) and C 6 H 12 O 5 (1,35×10 -10 mol). The corresponding product distributions at temperatures between 298-398 K are illustrated in Figures 2 and 3. Figure 2 shows the cellulose, glucose, formic acid and hydrogen distribution as a function of temperature whereas Figure 3 shows methane, water and carbon dioxide as a function of temperature.
The most significant finding from Figure 2 is that hydrogen increased exponentially as temperature increase within the stipulated temperature boundary. At temperature 298 K, 1.28×10 -5 moles of H 2 is produced and when the temperature reached 398 K, H 2 of 3.50×10 -4 moles is produced that is an increase of 27 times. In the other hand, the composition of cellulose, glucose and formic acid decreased insignificantly with increasing temperature. At 298 K, the composition of cellulose, glucose and formic acid are 1.35× -10 mol; 3.72× -10 mol and 5.85× -10 mol, respectively. Meanwhile, at T 398 K the amount was reduced to 1.98×10 -22 mol; 6.37×10 -22 mol and 22×10 -22 mol. It is assumed that decrease in cellulose, glucose, and formic acid take place because that substance was converted to H 2 . According to the insight on the reaction mechanism, hydrogen can be produced from cellulose by forming intermediate products including glucose, formic acid or other oxygenate products (Hao et al. 2018;Lan et al. 2020;Zhang et al. 2016). The more cellulose converted into organic compounds, the easier it is to for H 2 to be produced and this is the reason H 2 formation increased rapidly (Syaahidah et al. 2020). The conversion of cellulose into organic products begins with the dissociation of β-(1,4)-the glycosidic bonds in these polysaccharides into saccharides as their monomers. Such process of attaining hydrogen from biomass is complicated (Ahorsu et al. 2018). This dissociation stage is the rate limiting step for the formation of H 2 from cellulose. Furthermore, these saccharides can be transformed into other intermediate products such as  Products (mol)

Temperature(K)
alcohol or carboxylic acid or furfural and finally into hydrogen and other products within a complex reaction network (Lan et al. 2020;Puga 2016;Speltini et al. 2014;Syaahidah et al. 2020;Zou et al. 2018).
Other chemical compounds involved in the conversion of cellulose to hydrogen are methane, water, and carbon dioxide. From Figure 3, it is predicted that there is no change in their compositions. Methane is obtained as one of the main products in cellulose conversion to hydrogen. This gas may be produced from intermediate product that have alkyl group in their structure. Alkyl (CH 3 -) will react with proton (H + ) resulted CH 4 (Bahruji et al. 2011;Bowker et al. 2014;Hao et al. 2018) following reaction route: From the thermodynamic equilibrium point of view, H 2 was dominantly resulted at high temperature, while CH 4 was produced at low temperature. It is a challenge to eliminate this low hydrocarbon from this reaction. Researchers suggested to carry out methane steam reforming and successive water-gas shift reaction to obtain maximum yield of H 2 (Shimura & Yoshida 2011). The reformation of cellulose to hydrogen also produces large amount of CO 2 . CO 2 is produced by the oxidation of the alkyl groups of the intermediate products. However, CO 2 from biomass is considered more environmentally friendly than CO 2 produced from fossil fuels (Danish & Ulucak 2020; Koruba et al. 2017;Sher et al. 2020). Through photosynthesis, this CO 2 will be reversed reacted into cellulose (biomass) (Chen et al. 2019). In addition, CO 2 and CH 4 tend to produce if the intermediate product contains either glycerol or methanol (Tasleem et al. 2020). Furthermore, the produced CO 2 subsequently transformed to CH 4 and this is the plausible explanation of why CH 4 is largely produced. The proposed reaction mechanism of cellulose to hydrogen is illustrated in Figure 4.

RR'CHOH +H2O
RH +R'H-CO2 + H2 RR'CHOH +H2O RH +R'H-CO2 + H2 Prediction of product distribution in this study can be taken into consideration to increase the amount of hydrogen produced from the conversion of cellulose. Ideally, the reaction equilibriums can be adjusted by manipulating temperature for some objectives: (1) maximize H 2 production, (2) reduce greenhouse gases (CH 4 and CO 2 ) production, and (3) minimize carbon release (Cheng et al. 2019). The manipulating reaction also can be attempt by applied an appropriate catalyst to adjust H 2 production. The catalyst is expected to direct the reaction product so that the main product has a relatively high yield.

CONCLUSIONS
Hydrogen is feasible to be produced from biomass that contains cellulose. The value of ΔH r 0 ; ΔS r 0 and ΔG r 0 are +624,7500 kJ/mol; +2,1491 kJ/mol.K and -26,1540 kJ/mol, respectively. With a negative ΔG r 0 value, the forward reaction to produce H 2 from (C 6 H 10 O 5 )n will proceed spontaneously and exothermic under standard conditions. Analysis of equilibrium constant of this conversion has a large ln K value (> 1). It indicates that the formation of H 2 from (C 6 H 10 O 5 ) n can be plausible and feasible. With a negative ΔG r 0 value and large ln K, the product formation at equilibrium is strongly favored at equilibrium. The composition of the substances involved in (at 298K) from the largest to the smallest are CH 4 (4.5 mol), H 2 O (3 mol), CO 2 (1.5 mol), H 2 (1.28×10 -5 mol), HCOOH (5.85×10 -10 mol), C 6 H 12 O 6 (3,72×10 -10 mol), and C 6 H 12 O 5 (1,35×10 -10 mol). Interestingly, H 2 yield will increase significantly at elevated temperatures.